1. Field of the Invention
The present invention relates to a gate driving technology, more particularly, to an isolated gate driver adapted for, for example, a PWM-based switching power supply.
2. Description of the Related Art
A conventional isolated gate driver 10 as shown in FIG. 1 generally includes an isolated transformer T1, and can be used to drive the high power MOSFET Q connected with the secondary winding LS of the isolated transformer T1 in response to a driving pulse-width-modulation (PWM) signal VPWM. However, assuming that the turns ratio of the isolated transformer T1 is 1:1, so when the duty cycle of the driving PWM signal VPWM is smaller, the induced voltage VS (i.e. (1−D)*Vcc1, where D is the duty cycle of the driving PWM signal VPWM, and Vcc1 is the amplitude of the driving PWM signal VPWM) in the secondary winding LS of the isolated transformer T1 is larger and may be greater than the maximum gate-source voltage (Vgs_max) of the high power MOSFET Q.
On the other hand, when the duty cycle (D) of the driving PWM signal VPWM is sharply decreased, the induced voltage VS in the secondary winding LS of the isolated transformer T1 is sharply varied with the sharp variation of the induced voltage VP in the primary winding LP of the isolated transformer T1 due to the resonance generated from the primary side of the isolated transformer T1. In this case, the induced voltage VS in the secondary winding LS of the isolated transformer T1 may be greater than the maximum gate-source voltage (Vgs_max) of the high power MOSFET Q when the driving PWM signal VPWM is in the high-level. Moreover, the high power MOSFET Q may be mistakenly turned on when the driving PWM signal VPWM is in the low-level. Obviously, the high power MOSFET Q cannot be reliably driven by the conventional isolated gate driver 10.